Around 1995, Father Damian Zuerlein noticed a disturbing trend among his mostly Hispanic parishioners at Our Lady of Guadalupe church in south Omaha, Nebraska. When he went to shake the hands of his congregation at the end of the service, he noticed their grips were not as firm as they might have been. Some would wince; others would shift their torsos awkwardly so as not to trouble a painful shoulder; a few even had fingers missing. Since half of his flock were involved in some way in the meat-packing industry, it didn't take long for him to work out the root of the problem.

A century after Upton Sinclair wrote The Jungle, arguably one of the most politically influential American novels of the past 100 years, the ethnicity of the protagonists and the location of the story have changed, but the basic narrative of poor working conditions for immigrant labourers in the meat-packing industry and public concern over food quality remains constant. Working in the meat-packing industry is the most dangerous job in America.

The Jungle tells the story of a young Lithuanian couple, Jurgis Rudkus and Ona Lukoszaite, who move to Chicago's Packingtown at the turn of the century, where Jurgis finds work in the stockyards. At the outset Jurgis, full of youthful vitality and the optimism of a recent immigrant, goes to the gates of the stockyards and compares himself favourably to others waiting for work. "Do you want me to believe that with these arms" - and he would clench his fists and hold them up in the air, so that you might see the rolling muscles - "that with these arms people will ever let me starve?" But his American dream of self-reliance soon crashes on the rocks of American reality. Inside he sees how the rigours of the production line put those muscles to work.

He "watched the men on their killing beds, marvelling at their speed and power as though they had been wonderful machines ... The pace they set here, it was one that called for every faculty of a man, from the instant the first steer fell to the sounding of the noon whistle."

Cheated by swindlers and overwhelmed by bleak living and working conditions, low pay and poverty, he soon begins to resemble the "broken-down tramps" he saw on his first day at the gates. Sinclair's descriptions of conditions in the meat-packing industry are particularly shocking. Filthy meat is scraped off the floor and thrown into mixers and rendered into sausages. "This is no fairy story and no joke," wrote Sinclair. "The meat will be shovelled into carts and the man who did the shovelling will not trouble to lift out a rat even when he saw one." Over at the fertiliser plant workers occasionally fall in and get ground up with the rest of the meat.

Bad luck, corruption and ill health gradually wear the family away. Death and abandonment finish them. Jurgis becomes a tramp only to return to the slums, where he overhears a socialist orator while he finds shelter in a doorway. Eager to hear more, Jurgis becomes a convert to socialism.

Sinclair was an unlikely candidate to write a bestseller, and The Jungle was an unlikely work to become one. His previous books had produced little in the way of either fame or money, and he declared himself ready to make "a definite attempt to write something popular".

The magazine Appeal to Reason gave him a $500 advance, and with that he went to Chicago to record work in the meat-packing industry for seven weeks. According to his biographer, Leon Harris, access to down-at-heel workers was not a problem for Sinclair because he fitted right in.

"So shabby were [his] clothes that by no more elaborate a disguise than the carrying of a dinner pail and armed with a few simple lies appropriate to the area in which he was investigating, he had no trouble going everywhere and noting everything."

With its documentary prose style and polemical tone, the original manuscript was rejected by several publishers. A reader for Macmillan described it as "Gloom and horror unrelieved ... As to the possibilities of a large sale, I should think them not very good."

Doubleday finally bought it, and its success was instant. Translated into 17 languages within months, The Jungle sold 5,500 copies in just one day. "Not since Byron awoke one morning to find himself famous has there been such an example of worldwide celebrity won in a day by a book as has come to Upton Sinclair," wrote the New York Evening World.

Among its admirers was Winston Churchill, who wrote two essays praising the novel: "It pierces the thickest skull and most leathery heart." George Bernard Shaw gushed: "When people ask me what has happened in my long lifetime I do not refer them to the newspaper files and to the authorities, but to [Sinclair's] novels."

President Theodore Roosevelt read and was deeply affected by it, so much so that Sinclair embarked on a feverish correspondence with the president. Too feverish - in the end, Roosevelt wrote to Frank Doubleday, the head of the publishing house: "Tell Mr Sinclair to go home and let me run the country for a while."

The politicians were so interested because the book's release sparked a public outcry both nationally and internationally. When word got around that human beings could become an accidental ingredient in "Durham's Pure Leaf Lard", foreign sales of American meat halved that year. There was huge domestic pressure from consumers too, which in turn prompted demands from meat manufacturers to protect the integrity of their product. Sinclair's revelation that "They use everything about the hog except the squeal" did not sit well at the dinner table. Public revulsion over some of the scenes in the book sparked a rare event in American industrial history. The meat-packing industry implored the government for the kind of legislation that would renew confidence in meat.

Given Sinclair's socialist leanings, Roosevelt was suspicious of the veracity of his research. To confirm the findings, he sent the labour commissioner Charles Neill and social worker James Reynolds to make surprise visits to meat-packing houses in Chicago and write an independent investigation.

Their report concluded that the meat-packers of Chicago worked "under conditions that are entirely unnecessary and unpardonable and which are a constant menace not only to their own health, but to the health of those who use the food products prepared by them".

Before the year was out the Meat Inspection Act was passed, authorising the secretary of agriculture to inspect meat and condemn any found unfit for human consumption. On the same day, the Pure Food and Drug Act was passed, which prohibited the manufacture, sale or transport of adulterated food products or poisonous patent medicines. The latter paved the way for the Food and Drug Administration that still exists today, regulating everything from cosmetics to blood products.

It is difficult to think of a book, let alone a novel, that has forced the state to respond in such a comprehensive manner. And yet, while Sinclair was delighted with both sales and fame, it was not quite the response that he intended. He had dedicated the book to the "Workingmen of America" and had set out to make an emotional appeal to the nation over the plight of the working poor and the prospects of a socialist alternative. Instead he had generated a public panic about food quality. "I aimed for the public's heart," he wrote in his autobiography, "and by accident I hit it in the stomach."

The Jungle was very much a novel of its time - an era of mass migration, US military expansion and rapid economic and technological transformation. It earned its place in the US literary hall of fame not for its aesthetic qualities but for its practical effects. Thanks to its polemical style, formulaic narrative and, at times, propagandistic language, it has more currency as a work of literary journalism than of great fiction.

Those publishers who discarded the manuscript had underestimated not only the potential breadth of its appeal, but the political and journalistic context that made that breadth possible. Middle-class Americans, concerned that the concentration of capitalism in a few hands would leave them at the mercy of trusts and monopolies, began to revolt.

The social commentator Randolph Bourne described it as a period when "a whole people" woke up "into a modern day which they had overslept . . . they had become acutely aware of the evils of the society in which they had slumbered and they snatched at one after the other idea, programme, movement, ideal, to uplift them out of the slough in which they had slept".

These concerns gave birth to the Progressive movement, which found its literary expression in a more aggressive and socially responsive style of journalism. Roosevelt called these writers muck-rakers after the Man with the Muck-rake in John Bunyan's Pilgrim's Progress. The dreary realism of these writers "who could look no way but downward" made Roosevelt depressed. But as he himself became disenchanted with the Republican party and embraced the Progressive movement, so he came to hail their contributions and muck-raker became a term of endearment in praise of dogged, investigative journalism.

"There are, in the body politic, economic and social, many and grave evils, and there is urgent necessity for the sternest war upon them," he said. "I hail as a benefactor every writer or speaker, every man who, on the platform, or in book, magazine, or newspaper, with merciless severity makes such attack, provided always that he in his turn remembers that the attack is of use only if it is absolutely truthful."

Back at Our Lady of Guadalupe, Father Damian tried to raise the problems faced by the heirs of Jurgis Rudkus. He made an announcement at mass and the litany of complaints, both big and small, came in. Father Damian's anecdotal findings were backed up last year by a report entitled "Blood, Sweat and Fear" from the New York-based Human Rights Watch on conditions in the US meat and poultry industry.

"Nearly every worker interviewed for this report bore physical signs of a serious injury suffered from working in a meat or poultry plant," it argued. "Meat and poultry industry employers set up the workplaces and practices that create these dangers, but they treat the resulting mayhem as a normal, natural part of the production process, not as what it is - repeated violations of international human rights standards ... A century [after the publication of The Jungle] abusive working conditions and treatment still torment a mostly immigrant labour force in the American meat-packing industry."